Multitudes of kinds of leather-like sheets have been proposed as a ball covering or a non-slip covering requiring non-slip properties.
For instance, patent document 1 discloses a covering composition for imparting slip resistance to a surface of a base material, wherein the covering composition contains a polyurethane resin having a hydroxyl group in a molecule thereof, a liquid rubber having a hydroxyl group in a molecule thereof, an inorganic filler or an organic filler, and an isocyanate prepolymer. A cover layer made of the composition has a certain degree of water absorbability. However, if the water absorption amount is unduly increased, the non-slip performance of the cover layer may be lowered. Accordingly, in the case where the cover layer is frequently contacted with a human hand, the cover layer may be softened, and the touch of the cover layer may vary resulting from absorbing a large amount of sweat. In particular, for instance, in a condition that a basketball with the cover layer is continuously or continually used for a long time, and the cover layer is contacted with a large amount of sweat during a game, the above drawback is particularly serious.
Patent document 2 discloses a non-slip covering produced by impregnating and solidifying a resin having rubber viscoelasticity, with a needle-punched non-woven fabric being used as a core member, to form a foamed sheet-like member, and slicing the foamed sheet-like member at a position corresponding to an intermediate layer thereof, wherein a sliced surface of the non-slip covering has a porous configuration. Since the non-slip covering has a soft surface, and a small surface strength, the non-slip covering is likely to wear out. Also, because of a high tackiness, the non-slip covering has a poor durability in using as a covering for a ball such as a basketball.
Patent document 3 discloses a synthetic leather produced by; mixing gelatin to a synthetic rubber elastic material; subjecting the mixture to foaming while heating to obtain a foamed molded product; removing a part of a surface skin layer of the molded product; and removing the gelatin by hot water, wherein a porous structure is formed on a surface of the molded product. The above synthetic leather has drawbacks that the surface of the synthetic leather has a high tackiness, and a low apparent density. Accordingly, the synthetic leather has a low wear resistance, and a poor durability in using as a ball covering.
Patent document 4 discloses a leather-like sheet comprising; an entangled fiber sheet; a porous base layer made of a porous elastic material and a penetrating agent filled in the cavity of the entangled fiber sheet; and a porous outer layer formed on a surface of the porous base layer, wherein openings (microholes) having an average diameter from 50 to 100 μm are formed in a surface of the porous outer layer at a density from 300 to 10,000 openings/cm2, and the penetrating agent is filled in the openings. Because of multitudes of large openings in the entirety of the surface of the leather-like sheet, the apparent density of the leather-like sheet surface is likely to be reduced, with the result that the leather-like sheet is likely to wear out. Accordingly, in the case where the leather-like sheet is used as a ball covering, a long-time use of the ball may deprive the concave-convex configuration of the ball. Also, recesses of the leather-like sheet are likely to be stained. In the case where the recesses are stained, the stains are less likely to be removed. Further, the penetrating agent filled in the openings may be dissolved by sweat of the players during a game, with the result that the ball surface may be slippery.
Patent document 5 discloses a sweat-absorbing game ball, wherein a polyurethane wet coagulated cover layer is laminated on a surface of a fiber member containing polyurethane, the cover layer surface has projections, and recesses between the projections, and a plurality of openings are formed in side surfaces of the projections. The projection side surfaces may be stained by a long time use of the ball. In the case where the projection side surfaces are stained, the stains are less likely to be removed. Also, in the case the ball is stained, sweat absorbability may be lowered, and a natural leather-like touch may not be obtained.
Patent document 6 discloses a leather-like sheet for use in a ball, comprising an entangled fiber sheet, and a porous outer layer laminated on a surface of the entangled fiber sheet and having a concave-convex configuration, wherein surfaces of the projections of the porous outer layer have microholes (openings) with an average diameter from 5 to 100 μm, and surfaces of the recesses are substantially devoid of openings. The openings are formed by buffing the projection surfaces with use of a sand paper, a wire cloth, or a like tool, or dissolving the projection surfaces with use of a solvent. The openings to be formed by the above method may have an unduly large diameter. As a result, the apparent density of the ball surface may be reduced, and the ball is likely to wear out during a game. The wear may further increase the size of the openings, or diminish the openings. As a result, sweat absorbability may be lowered, and grip performance of the ball may be lowered. Also, an unduly large diameter of the openings is likely to cause stains on the ball surface.
Patent document 7 discloses a skin covering for a ball, comprising: a base layer, and a coat layer which is formed on a surface of the base layer, made of a porous polymeric elastic material, and has a concave-convex configuration, wherein openings of about 0.5 to 50 μm are formed in side surfaces of the projections at a density of 1,000 openings/cm2 or more. The openings are formed by; applying a surface treatment on a skin layer constituted of the porous polymeric elastic material with use of an organic solvent; subjecting the treated surface to an embossing treatment with a die having a concave-convex surface of 1 mm or more in height difference; and forming a cover layer made of a polymeric elastic material on top surfaces of the projections by coating. In use of a ball having a concave-convex surface with large pores solely in side surfaces of the projections, as described above, the projection side surfaces are likely to be stained, and the stains are less likely to be removed. Also, because the cover layer made of the polymeric elastic material is formed on the projection top surfaces, the touch of the ball may be degraded.    Patent document 1: JP No. Hei 7-30285B    Patent document 2: Japanese UM Publication No. Sho 63-197475A    Patent document 3: JP No. Sho 63-152483A    Patent document 4: JP No. 2000-328465A    Patent document 5: U.S. Pat. No. 6,024,661    Patent document 6: JP No. 2004-300656A    Patent document 7: JP No. 2004-277961A